Although they are not limited to the field of semiconductor technology, the present invention and the problems on which it is based are explained in relation to mould wafers.
In the field of semiconductor technology, disc-shaped workpieces, referred to in this case as wafers, are generally processed in a linear processing chain, i.e. in the context of flow line production in a plurality of successive devices which carry out coordinated method steps.
In the wafer processing in this case, exact alignment of the wafer is necessary before further processing, in particular before sawing out the individual chips from the wafer or before electrically contacting the chip. For this purpose, what are known as workpiece alignment devices are used, which are also known in the field of the semiconductor industry as wafer alignment systems.
From the state of the art, an alignment system is known for this purpose from U.S. Pat. No. 6,275,742 B1 for example, and can undertake exact alignment of a disc-shaped workpiece, in particular a wafer, by an optical method.
A device is known from JP 01-267403 A for determining the deflection of a disc-shaped workpiece, it being possible to determine the deformation at circumferential points of the workpiece using said device by means of an optical detector device.
Further devices and methods for determining a deformation of a disc-shaped workpiece, in particular of wafers, are disclosed in JP 10-078310 A, JP 06-163661 A, US 2006/0280085 A1, U.S. Pat. No. 4,750,141 A and U.S. Pat. No. 7,301,623 B1.
It is common to all these known methods that the deformation cannot be determined exactly, since the measurement methods or gripping devices distort the measurement result.
More recently in the semiconductor industry there has been a trend towards what are known as compound wafers or mould wafers, i.e. towards assembled, artificially produced wafers, which are generated by assembling individual chips into a wafer-shaped formation, the chips being glued into a disc-shaped structure again by means of a plastics material sealing compound. Mould wafers of this type, but also conventional thin wafers of silicon or the like, have a circular construction conditional on the previous thermal and mechanical working, and also exhibit deflection in the axial direction, in such a way that these disc-shaped workpieces are not planar, but deflected or deformed.
FIG. 4 is a schematic plan view of a mould wafer, having reference numeral 3, the plastics material moulding mass being denoted as 31 and the semiconductor chips embedded therein being denoted as 30. After the removal of a protective film, the semiconductor chips 30 are exposed at an upper side of the mould wafer 3.
Since the small dimensions of the individual semiconductor chips 30 necessitate exact positioning of the mould wafer 3 for subsequent processing steps, exact alignment must be provided, and knowledge of the deflection properties of the mould wafer 3 is required for this.
This leads to the problem of exactly determining the deflection or deformation of a disc-shaped workpiece of this type. This problem is all the more pressing given that the mould wafers 3, which represent an assembly of a number of individual chips into a wafer-shaped construction using a sealing compound, have a high inherent curvature as a result of the different thermal expansion coefficients of silicon and plastics material, in such a way that no mould wafer 3 is the same as any other. Thus, exact alignment is only possible taking into account the individual deflection of the mould wafers 3, so as to make linear processing possible on a large scale in the manner of an assembly line.
A limit may also be placed on the deflection, so as to discard mould wafers 3 which cannot be processed further.